Abstract : The process and main points of pulsed MAG mixed gas shielded welding in the manufacture of membrane wall are introduced. It is proved that the process described in this paper has good manufacturability, and the membrane wall produced can meet the welding quality requirements well.

Key words : membrane wall, pulsed MAG welding

0 preface

The light pipe + flat steel welded membrane water wall has good air tightness, reduces air leakage, makes the furnace can be burned with micro-positive pressure, the furnace wall only needs to use light insulation material and is convenient to adopt suspension structure and easy to assemble and install. It is widely used in water-cooled wall structure types of power station boilers at home and abroad. The manufacturing quality and dimensional deviation must meet the requirements of JB/T5255-91, and the advantages and disadvantages of the manufacturing process are clearly a crucial part. The membrane wall manufacturing of the boiler industry mainly adopts two methods: submerged arc automatic welding and MAG gas shielded welding. In the 1980s, submerged arc automatic welding was widely used due to its deep penetration, high welding quality, and no arc radiation. However, due to its large welding deformation and the inability to form a short and positive shape, it is being matured by MAG gas. Protective welding impact. MAG gas shielded welding can realize the positive and negative one-time forming of the weld seam, the deformation of the pipe screen is small, the weld bead is full, and the production efficiency is high. Moreover, due to the use of the argon-rich mixed gas, the arc combustion is stable, the splash is small, and the arc axial force is large. The problem of large surface tension of the arc drifting is overcome, the penetration depth is curved, the metal fluidity is good, and the dense weld seam is easily obtained. The wire utilization rate has been improved and the working conditions have been improved, and thus it is being used more and more widely.

1 Welding equipment and main parameters

Our factory currently has two specifications MAG membrane wall welding machine, the main parameters are as follows:

1.1 3200×8 pole membrane wall MAG gas shielded welder

Maximum workpiece width: 3200

Maximum length of workpiece: 12m

Roller height: 225mm

Welding torch lateral adjustment range: 1250mm

Suitable welding pipe specifications: φ32~φ76 wall thickness 3.5~9mm

Suitable for welding flat steel specifications: width 12~110mm, tolerance ±0.1, thickness 4~9mm

Pipe and flat steel: low carbon steel and low alloy steel

1.2 1600×12 pole membrane wall MAG gas shielded welder

Maximum workpiece width: 1600

Maximum length of workpiece: 12m

Roller height: 170mm

Welding gun lateral adjustment range: 1000mm

Suitable welding pipe specifications: φ32~φ76mm, wall thickness 3.5~9mm

Suitable for welding flat steel specifications: width 12~110mm, tolerance ±0.1, thickness 4~9mm

Pipe and flat steel: low carbon steel and low alloy steel

1.3 Power supply

The welding uses the Finnish Kenby inverter welding machine. The specific models are PRO4200 welding power supply, PROMIG501 wire feeding machine; PROCOOL30 water tank; pen water cooling gun (420AmP) and other matching components to ensure reliable welding operation and good performance.

2 Welding process parameters and adjustment:

2.1 Parameter range

Welding wire: SM-70, φ1.2

Polarity: DC reverse connection

Welding current and voltage:

Upper station welding torch: 200-220A, 25-27V

Lower station welding torch: 180-200A, 25-27V

Welding speed: 650-700mm/min

Dry wire elongation: 15±1mm

Gas flow rate: 15-20 l/min

Mixing gas ratio: Ar (80-85): CO2 (20-15)

Welding torch anteversion angle α: 00-150

Torch lateral inclination angle β: 250-300

2.2 Main parameter adjustment

2.2.1 Wire dry elongation L

L is too long, and it is easy to be overheated when the current is constant. The spatter is severe, causing the welding process to be unstable, and the distance between the nozzle and the workpiece is also increased, and the gas protection effect is deteriorated.

Of course, if L is too small, the distance between the nozzle and the workpiece will be shortened, and the splash metal will easily block the nozzle.

2.2.2 torch forward angle α

As the α angle increases, the spatter rate decreases, the melt width increases, the residual height decreases, and the weld edge edge fusion performance is greatly improved, which is advantageous for eliminating the undercut phenomenon.

2.2.3 Torch lateral inclination β

If the angle of β is too large, the meltability of the side of the weld tube will be deteriorated. At the same time, arc interference will occur in the same group of torches, and the splash will increase. Therefore, the β angle should be small.

2.2.4 Welding current, voltage and welding speed

Larger current should be used for welding, and the corresponding welding speed and voltage should be matched to improve the arc stiffness, reduce the influence of magnetic partial blow, and ensure the weld penetration and size requirements. Appropriate matching of the pulse specification parameters allows for extensive adjustment and precise control of the weldment input. At the same time, attention should be paid to the uniformity of the heat input between the adjacent arcs and the symmetry of the welding position. In practice, this is found to be a effective measure to effectively reduce the side bend.

2.2.5 Mixing gas ratio: As the Ar gas content increases, the droplet transfer frequency increases and reaches a maximum value near the 80% Ar gas content; and the gas ratio also has a great influence on the splash, and the Ar gas content increases. The spatter rate decreases, and the 75%~85% Ar gas content decreases the fastest, about 2%~3%.

3 Membrane wall assembly process points

3.1 Tile type

3.1.1 Basic unit

The basic unit O-O-O, -O-O-, -O- can be realized on both welding machines;

O-O-O-O, -O-O-O- is mainly realized on a 1600×12 pole welder.

3.1.2 Small pieces

According to the size and width of the product, different basic unit chips are selected for the piece, and the welding is mainly performed on the 3200×8 pole welding machine.

3.1.3 Big piece

The small piece is further assembled into a tube screen that meets the total width requirement of the product, and is mainly welded on a 3200×8 pole welding machine.

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